The most common physical instability in bottled wine observed by the AWRI’s helpdesk is the precipitation of the potassium salt of tartaric acid, potassium hydrogen tartrate (also known as potassium bitartrate or KHT), which is generally associated with inadequate cold stabilisation of wines. It is therefore considered essential that steps are taken to help prevent post-bottling KHT precipitation, particularly in white winemaking, as crystalline deposits are not well accepted by consumers.
Cold stability can be described as the tendency of a wine to resist precipitation of KHT crystals when exposed to low temperature. Calcium tartrate (the calcium salt of tartaric acid or CaT) can also precipitate to form a crystalline deposit; however, low temperature does not have as great an effect on CaT precipitation as it does on KHT precipitation, so it can be difficult to stabilise a wine with respect to CaT. As such, methods of cold stabilisation are typically designed to achieve a level of stabilisation with respect to KHT.
The cold stability of wines should be tested prior to packaging, particularly white wines, to give a prediction about whether or not KHT will crystallise and precipitate after packaging.
Cold stability tests
There are a number of tests available to check a wine’s cold stability, including:
- Concentration product
- Saturation point (Tsat)
Details about these tests are provided on the Measurement of cold stability page. However, it must be remembered that there is no ‘universal’ definition for cold stability. Cold stability, or KHT stability, is a relative term which can be defined differently by different producers. Note that the AWRI recommends the ‘brine test’ where a filtered sample of the wine is held at −4⁰C for three days and then checked for any crystalline deposit. In a trial where four common cold stability tests were studied, the results of this test corresponded most closely with the observation of the stability of wines stored under cellar conditions for one year (Leske et al. 1996). Unlike the results for the other three tests (freeze/thaw, conductivity and concentration product), no wines that were indicated to be stable by the ‘refrigeration/brine’ test were subsequently found to have a crystalline deposit.
Current stability vs potential stability
The cold stability tests listed above test a wine’s current stability, with the exception of the Saturation point test (Tsat), which is a measure of the wine’s potential to become unstable in the future. This relates to the presence of crystallisation inhibitors, which are naturally occurring components of wine (such as polysaccharides and mannoproteins) that help prevent microscopic KHT crystals from growing to a size where KHT crystallises out of solution and forms a deposit. As a wine matures or undergoes winemaking processes the levels of these inhibition compounds can change, potentially allowing tartrate to crystallise and deposit. This can happen even after traditional cold stabilisation has occurred. Consequently, there are essentially two types of tartrate stability:
- ‘Current stability’ is a measure to show if the wine will precipitate tartrates ‘here and now’ if chilled.
- ‘Potential instability’ is a measure of the wine’s potential to become unstable as the wine loses crystallisation inhibitors (ages/changes), even if it does not currently precipitate crystals when chilled.
A combination of tests is a good option when it comes to testing cold stability
The most reliable information can come from a combination of tests, giving results for both ‘potential’ and ‘current’ stability. By taking this approach it is possible not only to determine a longer term indication of a wine’s cold stability risk, but also to help determine the most appropriate strategy to treat a wine to achieve a tartrate-stable product. The lowest risk is when a wine passes both ‘potential’ and ‘current’ stability tests.
Cold stability of sparkling wines
Sparkling wine producers stabilise their base wines to prevent KHT precipitation in the final sparkling wine after secondary fermentation by using a modified version of the cold stability test. Realising that secondary bottle fermentation will add 1-1.5 % alcohol, which will affect the wine matrix and thus cold stability, they fortify a small quantity of the cuvee and perform a cold stability test on the fortified sample.
Some reasons why a wine might fail a cold stability test even though the wine was previously ‘stabilised’
- The cold stability was not checked or confirmed with a reliable test after cold stabilisation
- The wine was allowed to warm up before racking/filtering off tartrate lees after stabilising (and stability was not checked with a reliable method afterwards)
- There was insufficient filtration after stabilising (and stability was not checked with a reliable method afterwards)
- The wine was blended after cold stabilising (and stability was not checked with a reliable method afterwards).
Options if a wine fails the pre-packaging ‘brine’ cold stability test
If a wine is sent for packaging, then it should already be cold stable. However, if for some reason a wine fails the pre-packaging ‘brine’ cold stability test but the fail is only ‘slight’ (i.e. only a small number of crystals are observed during the test), then the winemaker could consider using a crystallisation inhibitor. Crystallisation inhibitors work by binding with one or more of the crystal faces after nucleation, preventing further growth and the appearance of visible crystals (Lankhorst et al. 2017). Note that cold stability testing methods that measure potential instability (e.g. the Tsat test) tend not to show the impact of crystallisation inhibitors.
Crystallisation inhibitors that can legally be added to wine in Australia (Anon. 2019) are summarised below. Note that these are rarely used without some form of prior cold stabilisation, and are often used as an extra guarantee of stability for higher risk wines. Some winemakers may also use electrodialysis to remove potassium and tartrate ions. Cold stability should be re-tested after the treatment (using a test that measures ‘current stability’) to ensure the process was successful.
Carboxymethyl cellulose (CMC)
Carboxymethyl cellulose (CMC) is a linear-chain polysaccharide molecule comprised of glucose units, with some of the hydrogen atoms of the hydroxyl (-OH) groups on the glucose units of cellulose replaced with sodium carboxymethyl (-CH2-COONa) groups. CMC is water-soluble and has no known harmful effects on human health (Su et al. 2010). Addition of CMC to wine does not cause any change in pH or titratable acidity and does not generally have any sensory impact (Guise et al. 2014, Salagoïty et al. 2011, Salamone and Oberholster 2015).
A range of CMC products are available and, due to the variability in the polymer length and number of side groups, their effectiveness can vary somewhat. Consequently, dosing rates to achieve stability may vary from one product to another, so it is best to follow the manufacturer’s instructions and allow two to five days for the CMC to integrate fully with the wine before any filtration (Wilkes et al. 2012). While there is no specified maximum limit for CMC in Australia, it should be used in accordance with good manufacturing practice (Anon. 2016) and limits may exist in some export markets. Note that CMC should not be relied upon to stabilise a grossly cold-unstable wine; its use prior to bottling should be limited to wines that have undergone cold stabilisation but are found to be just unstable prior to bottling.
Key points for CMC use:
- Works well for white wines
- Not suitable for red wines (will affect colour, but might be suitable for some rosé wines)
- Wine must be already protein stabilised before use
- Must be used after fining but before final filtration
- No additions can occur after addition of CMC
- Has potential to affect filterability (need to allow 2 to 7 days post-treatment before filtration)
- Not effective against calcium tartrate precipitation
- Maximum dosage of 100 mg/L
- Tends not to work on wines that are grossly unstable.
Further information on CMC can be found in the FAQ on carboxymethyl cellulose.
Metatartaric acid is a mixture of polymers with different molecular weights formed by esterification of tartaric acid (Marchal and Jeandet 2009). The effectiveness of different metatartaric acid products may vary, depending on the average esterification rate (Ribéreau-Gayon et al. 2006). Although metatartaric acid is effective at inhibiting crystallisation, its effects are not stable over time as it undergoes hydrolysis, leading to an increase in acidity due to the release of tartaric acid (Marchal and Jeandet 2009). Consequently, metatartaric acid loses its inhibitory effect over time, depending on wine storage temperature, with more rapid hydrolysis occurring at higher temperature.
Ribereau-Gayon et al. (2006) indicate that metatartaric acid can be considered effective for the following time periods, depending on temperature:
- Several years at 0°C
- Over two years at 10–12°C
- One year to eighteen months at temperatures varying between 10°C in winter and 18°C in summer
- Three months at 20°C
- One month at 25°C
- One week at 30°C
- A few hours between 35 and 40°C.
Given warmer temperatures increase the rate of hydrolysis, metatartaric acid should be prepared in cold water just before it is to be used and should be added after any fining, but before the final clarification. Note that metatartaric acid is highly hygroscopic (i.e. it absorbs moisture) so it should be stored in a dry place (Ribereau-Gayon et al. 2006). While there is no specified maximum limit for metatartaric acid in Australia, it should be used in accordance with good manufacturing practice (Anon. 2016) and limits exist in some export markets, such as the European Union.
Mannoproteins are complex polymers consisting largely of mannose (a type of sugar) and certain proteins. They are released from yeast cell walls during autolysis (Marchal and Jeandet 2009). Mannoproteins can be extracted from yeast cell walls using enzymes and ultra-filtration to produce a purified commercial product, which can inhibit KHT crystal growth without affecting a wine’s organoleptic properties (Moine-Ledoux and Dubourdieu 1999, Moine-Ledoux et al. 2002).
Gerbaud et al. (2010) found that yeast mannoproteins effectively inhibited KHT crystal growth at a concentration of 200 mg/L for some wines; however, a higher concentration was required for wines that were highly saturated with KHT. While addition rates up to 250 mg/L appear to inhibit KHT crystal growth in white, rosé and red wines (Moine-Ledoux and Dubourdieu 1999), an addition rate of 300 mg/L in some wines may cause mannoprotein to flocculate (Gerbaud et al. 2010, Moine-Ledoux et al. 2002). Due to differences in the age, processing history and colloidal nature of different wines, the addition rate of a mannoprotein product should be determined by benchtop trials before addition to the bulk wine (Ribereau-Gayon et al 2006). Bower (2007) indicates that a wine must be assessed to determine its suitability for a mannoprotein addition, with such an assessment involving the measurement of parameters such as wine turbidity and filterability.
Suppliers of mannoprotein advise that average doses are between 100 and 300 mg/L and that the addition should occur between preparation filtration and bottling filtration, at the latest the day before bottling (Anon. 2020). Note that mannoproteins are not as effective at inhibiting calcium tartrate crystal growth as they are against KHT (Anon 2020). As with the crystallisation inhibitors discussed above, mannoproteins should be used in accordance with good manufacturing practice (Anon. 2016).
Key points regarding the use of mannoproteins:
- Important to conduct bench trials (as close to bottling as possible) before adding mannoproteins to wine
- Should be added after fining and pre-filtration, as these processes may remove mannoproteins
- Not suitable for wines supersaturated with KHT, as high addition rates would be required, which introduces the risk of product flocculation
- Not effective against calcium tartrate precipitation
- May react with other wine components over time and lose effectiveness.
Potassium polyaspartate (KPA)
Polyaspartates are polyamides synthesised by thermal polymerisation of the amino acid L-aspartic acid. Low molecular weight polyaspartates were demonstrated in research studies to be effective inhibitors of KHT crystallisation in white and red wines without affecting red wine colour (Bosso et al. 2015). Asproudi et al. (2015) also found polyaspartates to be effective inhibitors of KHT crystallisation in wines with different physicochemical composition. More recently, Bosso et al. (2020) investigated the effect of different doses of KPA on the tartrate stability of six wines (three white and three red) after six and 12 months of bottle ageing, as well as the effect on filterability and the colour of the white wines. These authors found a 100 mg/L dose was sufficient to stabilise all the wines, which remained stable by a cold test (−4°C for six days) after one year of bottle ageing. There was no effect on white wine colour and the KPA additions did not significantly modify red or white wine filterability using 0.45 μm filters (Bosso et al. 2020). Scrimgeour et al. (2020) also recently investigated the effect of KPA on the tartrate stability of 13 wines (eight white, two rosé and three red). These authors found that all the KPA-treated wines passed the ‘brine test’ after three months and were even stable when the test was extended for 20 days (instead of the usual three days).
It is best to follow manufacturer’s instructions, but KPA is typically added just before the final filtration (Bosso et al. 2020). As with the other crystallisation inhibitors, KPA should be used in accordance with good manufacturing practice (Anon. 2016).
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